a) Field of the Invention
The present invention is concerned with a self-fluxing binder composition for use in the pelletization of ore concentrates, preferably iron ore concentrates.
The invention is also concerned with the green, dry or fired ore concentrate pellets produced with such a self-fluxing binder composition.
b) Brief Description of the Prior Art
With the depletion of high quality natural metallic ores, most of the ores which are presently extracted, are usually subjected to significant size reduction and subsequent upgrading by mineral beneficiation process prior to being used. The upgraded metallic ores that are so obtained, are usually in the form of very fine particles or may be ground as such to make them amenable to a technique called "agglomeration", which also has been successfully applied in a wide variety of industries such as fertilizer, chemicals, nonmetallic minerals, industrial wastes etc. . . As a matter of fact, agglomeration is the most important technique applied for the treatment of fine iron ore concentrates, which otherwise cannot be easily handled, shipped or charged to a blast furnace or for the production of direct reduced iron.
Although agglomeration can be achieved in many ways, sintering, pelletizing and, to a lesser extent, briquetting are the most important agglomeration processes. While sintering requires relatively coarse particles (lower than 0.2 mm), pelletizing can generally be achieved with particles lower than 0.2 mm, and preferably finer.
Pelletizing is a well known process which comprises essentially two basic operations:
1) preparation of green balls or pellets, and PA1 2) firing of the green pellets through a number of successive steps including drying, preheating and firing. PA1 Traice, F. B. and Lappin R. L. "Development and in ironmaking practice, proc. conf. 22-23, Nov. 1972, Iron Steel Ins.) London p. 17-31; PA1 Biswas A. K. "Principles of blast furnace iron making", Cootha publishing house, Brisbane Australia, 1981, p. 206.
The preparation of good quality green pellets is a prerequisite for the production of high quality fired pellets. Indeed, the green pellets must have adequate strength properties and impact resistance to withstand handling, resistance to a number of drops, drying and eventually firing in the subsequent stage.
To achieve such a goal, it is of conventional practice to add a binder to the ore concentrate prior to its pelletization in order to ensure that the produced green and dry pellets have adequate strength properties for subsequent handling, transportation and heat hardening treatment. Binders are known to affect both physical and metallurgical quality of the pellets. Accordingly, any binder to be actually useful, must ensure adequate performance during the formation of the green pellets and during their firing, and also impart good pellet properties.
So far, bentonite, which is an aluminum silicate clay, has commonly been used as a binder. Bentonite forms permanent bridges between iron oxide grains during firing blocking access to internal pore structure. Its composition may substantially vary from one extraction site to another and its use contributes to a significant increase in the gangue content of the pellets because of its high concentration of silica and alumina.
Pellets, especially iron ore pellets, produced with use of bentonite as a binder, usually require additional basic fluxes containing CaO and MgO when they are produced or during their use, i.e. when they are fed into a blast furnace, to meet some standard metallugical requirements as well for production of metal of good quality. Thus, the addition of extra fluxing materials contributes to the production of higher slag volume in the blast furnace, requiring extra energy that adds to the operating cost of the blast furnace
Therefore, considerable efforts have been made in the past and are presently being pursued to reduce the bentonite consumption in pelletizing or to find a substitute for it.
Peridur.RTM., a dry cellulose based organic binder (U.S. Pat. Nos. 4,288,245 and 4,597,797) has been proposed as such a substitute. Although Peridur.RTM. is efficient, its high cost and certain deterioration noticed in the metallurgical qualities of the pellets produced with it, has restricted so far its use in a great majority of the pelletizing plants
As a matter of fact, despite the fact that bentonite has several disadvantage, no other binder has been found so far to replace it. Moreover, because it has almost exclusively been the only binder used during the entire growth span of the pelletizing industry, bentonite has created certain standards in the iron ore pelletizing industry. Therefore, any new binder to be accepted must meet these standards, in addition to providing the pellets with the basic strength properties required for transportation, storage and mixing procedures. Above all, any new binder to be accepted must be both energy and cost efficient. Any new binder to be accepted should also address a number of important issues imposed by environmental protection agencies. Moreover, large scale production of such a binder had to be both economically and technically feasible over a period of time.
Peridur.RTM. and other similar type of organic binders like those disclosed in U.S. Pat. Nos. 4,863,512 and 4,948,430, although they reduce the chemical "contamination" of the ore concentrate with unwanted elements do not address alone all the requirements of pellet quality. In order to meet these pellet quality requirements, additional materials, especially basic fluxing materials, must be added to the concentrate. It is essentially because of the above limiting factors and because of their high cost that the use of such substitute binders have severely been restricted in a large majority of pelletizing plants.